13 Sep Increasing the throughput of the electrospinning process

Multi-jet technology is a necessary approach to increase the throughput of the electrospinning process. Although multi-jet electrospinning is a more complex process than the single-jet, it has been demonstrated to be a better approach to enhance electrospinning productivity rather than substantially increasing the throughput of a single spinneret. Modified single-nozzle, multi-nozzle and needleless systems, have been developed to obtain multiple jets and thus to increase the system throughput.

Multi-jets electrospinning was first accomplished using a single nozzle with a grooved tip, from the branches of which multi-jets were formed. This multi-cone technique proved to improve the productivity of the single-nozzle electrospinning process. However, the process is rather chaotic due to the poor control of the polymeric solution flow rate through each of the cones generated. Further, the mechanism behind fiber formation has not been examined. Later, the use of curved collectors on single-nozzle configuration to generate multiple Taylor cones was proposed as an alternative to enhance the throughput. Two possible mechanisms for this phenomenon were provided including electric field distribution and clogging of the passageway of the polymer solution. However, there are still a number of unsolved issues related to electrospun webs, including fiber morphology and diameter. No public reports have been to date published on mass production of nanofibers by this approach.

An alternative procedure considered to increase the throughput of the single-nozzle was to split the polymer jet into two separate sub-filaments during its flight to the fiber collector by applying a sufficiently large tangential stress.

The most direct method to increase the electrospinning injector throughput relies on the use of a bundle of nozzles. Nozzle configuration, nozzle number and nozzle spacing are the three key parameters to design a multi-nozzle injector. The multi-nozzle injector can be organized into: linear arrays and two-dimensional arrays, i.e. square, circular and elliptic, hexagonal and triangular.

A linear array of nozzles is the simplest multi-nozzle configuration. Several examples have been reported in the last few years. For example, linear multi-nozzle electrospinning setups composed of four nozzles were designed to produce nanofibers. It was shown that the nanofibers were unevenly deposited onto fibrous substrates probably because of the distorted electric field effect due to the finite boundary conditions. Further, spinnerets with arrays of seven and nine nozzles were employed to examine the behaviour of jets in multi-nozzle electrospinning.

Experimental results as well as simulations showed a contrasting behaviour between outer and inner jets such as bending direction and envelope cone. However, each jet was subjected to the typical bending instabilities observed in single-jet electrospinning. Electric field shielding at the inner nozzles was observed in a nozzle injector with a linear arrangement. It was observed that only the outer nozzles were active whereas the inner ones did not generate the Taylor cone.

An additional electrode is necessary in order to compensate for the boundary effect and homogenize the overall electric field. Most investigations on multi-nozzle electrospinning have been focused on two dimensional arrays. For example, multi-nozzle injectors with elliptic and circular configurations were designed to improve the process stability and throughput. It was shown the circular configuration was a more stable process and obtained higher a throughput for polyvinyl alcohol (1 versus 0.4 mg/min per nozzle).

Other multi-nozzle devices have been designed and developed, increasing the throughput obtained significantly. Some of them are able to mass-produce composite nanofibers webs from several kinds of polymer solutions. More recently, an industrial multi-nozzle injector composed by 1000 nozzles was reported.

At our company Bioinicia, and particularly through our engineering division Fluidnatek®, we have launched since 2012 a range of high-throughput multi-nozzle equipment for both pilot plant and manufacturing purposes called LE-500 and LE-1000, respectively. These tools that come with climate control unit for temperature and relative humidity control are designed to, for instance, meeting the existing stringent legislation criteria in the bio space and to provide complete flexibility for the manufacturing of electrospun / electrosprayed products. The equipment was designed to be integrated in industrial production lines involving roll-to-roll collection or any other of collection, pre or post-processing steps. Bioinicia has also built two demonstration/contract manufacturing electro-hydrodynamic processing plants in Valencia, Spain, that are currently capable of manufacturing a minimum of 2 T a year of nanofiber-based products with pharma/biomedicine legislation compliancy (GMP compliance) and a minimum of 1 kg/hour of powder-based products for pharmaceutical, cosmetic, agrochemical, nutraceuticals and food applications.